1. Field of the Invention
The invention relates to an image pickup apparatus for picking up an object image.
2. Related Background Art
In recent years, attention has been paid to a solid-state image pickup element called a CMOS sensor using a CMOS process. The application of the CMOS sensor to, particularly, the field of portable information apparatuses has been progressed because of easy fabricating together with peripheral circuits, low-voltage driving, and the like. At the initial stage of research of the CMOS sensor, fixed pattern noises, which are generated due to a variation in element characteristics of each pixel were a large problem. As one of methods of effectively eliminating the fixed pattern noises, the following method using a clamping circuit has been proposed. FIG. 7 shows an equivalent circuit diagram of the solid-state image pickup element according to the prior art. FIG. 7 shows a portion regarding one certain pixel (it is assumed to be a pixel arranged at the nth row and the mth column here) among pixels which are two-dimensionally arranged. In FIG. 7, a unit pixel comprises: a photodiode 1 as a photoelectric converting element; an amplifying MOSFET (Metal Oxide Silicon Field Effect Transistor) 4 for amplifying a signal generated in the photodiode 1; a pixel reset switch 3 for resetting an input of the amplifying MOSFET 4 to a predetermined electric potential; and a row selecting switch 5 for controlling electric connection between a source electrode of the amplifying MOSFET 4 and a vertical output line 6. When the row selecting switch 5 is turned on, a source follower circuit in which a load MOSFET 7 is used as a load is formed and an electric potential reduced from an electric potential of the photodiode by an amount corresponding to a threshold value of the amplifying MOSFET 4 appears on the vertical output line 6 in accordance with the electric potential of the photodiode 1. A clamping capacitor 8 and a clamping switch 9 construct a clamping circuit. The clamping circuit shuts off a DC component of the vertical output line potential and transfers only a potential change amount based on a light charge amount to the post stage. A light response component obtained via the clamping circuit is temporarily held in a holding capacitor 12. Thereafter, it is read out to a horizontal output line 17 via a horizontal transfer gate 14 which is controlled by a horizontal scanning circuit 19. The horizontal output line 17 is connected to an input of an output amplifier 18. The output amplifier 18 drives an external load (not shown).
The operation of the solid-state image pickup element will now be described in detail by using driving pulse timing in FIG. 8. It is assumed that prior to the reading operation, a predetermined exposing time has elapsed and photo-charges have been accumulated in the photodiode 1. With respect to the nth row selected by a vertical scanning circuit (not shown), a row selecting pulse PSEL is set to the high level and a light output according to the light charge amount appears on the vertical output line 6. A subscript “n” added to each of notations “PSEL” and “PRES” in FIG. 8 indicates a pulse which is applied to the pixel of the nth row. Subsequently, a clamping pulse PCOR changes from the high level to the low level and the light output is clamped. At this time, an electric potential of the holding capacitor 12 has been reset to a clamping voltage VCOR. Subsequently, the pixel reset pulse PRES is held at the high level for a predetermined period of time and when the photodiode 1 is reset, a dark output appears on the vertical output line 6. At this time, a change amount from the light output to the dark output is transferred as a photosignal component to the holding capacitor 12 via the clamping capacitor 8. At this time, assuming that a capacitance of the clamping capacitor 8 is CO and a capacitance of the holding capacitor is CT, then a photosignal component is transferred with a gain of CO/(CO+CT). When a transfer pulse PTS changes from the high level to the low level, a circuit between the clamping capacitor 8 and the holding capacitor 12 is shut off and a light response component is held in the holding capacitor 12. Fixed pattern noises, which are generated due to a variation in threshold value of the amplifying MOSFET 4 of each pixel have been eliminated by the clamping circuit constructed by the clamping capacitor 8 and the clamping switch 9. After that, scanning pulses HSR(1) . . . HSR(m) are generated by the horizontal scanning circuit 19, the horizontal transfer gate 14 is sequentially opened/closed, and the photosignal temporarily stored in the holding capacitor 12 is read out to the horizontal output line 17 (for example, Japanese Patent Application Laid-Open No. H04-61573).
However, the above-described solid-state image pickup element according to the prior art has the following problems. When the clamping switch 9 is turned off, since a part of inverting layer charges formed in the clamping switch 9 flows into the holding capacitor 12 side, the electric potential after the turn-off of the switch 9 drops to a level lower than the clamping voltage VCOR. Since a parasitic capacitance exists between a gate and a source (drain) of the clamping switch 9, the clamping pulse PCOR is mixed into the holding capacitor 12 side via the parasitic capacitance. If an amount of inverting layer charges and the parasitic capacitance are different for every clamping switch, a difference between offset voltages occurs and the fixed pattern noises are generated in the horizontal direction. Generally, there is a tendency that the farther a distance between two different switches is, the larger the difference between the offset voltages is, and a phenomenon that an output level changes gradually in the horizontal direction occurs. Such a phenomenon results in a luminance fluctuation of a display screen. In the case of a color sensor, a spatial deviation of a white balance is further caused. Since the offset voltage fluctuates at random due to an event that the clamping switch is turned off, it becomes a factor of generation of random noises. By taking into consideration of a fact that a transfer gate 10 is ON when the clamping switch 9 is turned off, an amount of such random noises is expressed by√{square root over ((kT/(CO+CT))}where,                k: Boltzmann's constant        T: absolute temperature        
Particularly, the higher definition an image sensor has, the less the values of CO and CT become in association with a decrease in pixel pitch. Therefore, there is a tendency of an increase in random noises. As mentioned above, offset components which are generated in the clamping switch 9 become the fixed pattern noises and random noises to deteriorate picture quality, and become an obstacle to realization of high picture quality by the solid-state image pickup element and its driving method of the prior art.